Olefin (co)polymers such as polypropylene and polyethylene are widely used in a variety of molding fields because of their excellent mechanical properties, chemical resistance and cost-effectiveness. Conventionally, the olefin (co)polymers generally have been produced by (co)polymerizing olefin by using a so-called Ziegler-Natta catalyst, which is obtained by combining titanium trichloride or titanium tetrachloride, or a transition metal catalyst component comprising titanium trichloride or titanium tetrachloride supported by a carrier such as magnesium chloride, and an organic aluminum compound.
In recent years, on the other hand, a catalyst that is obtained by combining metallocene and aluminoxane, which is different from catalysts in the prior art, is used to (co)polymerize olefins to obtain olefin (co)polymers. The olefin (co)polymer obtained by using the metallocene-based catalyst has a narrow molecular weight distribution, and in the case of copolymers, comonomers are copolymerized uniformly. Therefore, it is known that more homogeneous olefin (co)polymers can be obtained than in the prior art. However, compared with olefin (co)polymers obtained by using a conventional catalyst type, the olefin (co)polymers obtained by using the metallocene-based catalyst have a lower melt tension, so that they are not suitable for some uses.
In order to enhance the melt tension and the crystallization temperature of polypropylene, the following methods have been proposed: a method of reacting polypropylene with an organic peroxide and a crosslinking assistant in a molten state (Japanese Laid-Open Patent Publication (Tokkai-Sho) Nos. 59-93711, 61-152754); and a method for producing gel-free polypropylene with free-end long chain branching by reacting semi-crystalline polypropylene with a peroxide having a low decomposition temperature in the absence of oxygen (Japanese Laid-Open Patent Publication (Ibkkai-Hei) No.2-298536).
Other methods for enhancing melting viscoelasticity such as melt tension have been proposed, such as a method of using a composition comprising polyethylenes or polypropylenes having different intrinsic viscosities or molecular weights, or producing such compositions by multistage polymerization.
Examples of such a method include a method in which 2 to 30 parts by weight of ultra high molecular weight polypropylene are added to 100 parts by weight of ordinary polypropylene and extrusion is performed in a temperature range from a melting point to 210.degree. C. (Japanese Patent Publication (kko-Sho) No. 61-28694), a method using multistage polymerization to obtain an extrusion sheet formed of two components of polypropylene having different molecular weights and a limiting viscosity ratio of at least 2 (Japanese Patent Publication (Ibkko-Hei) No. 1-12770), a method of producing a polyethylene composition formed of three types of polyethylene having different viscosity average molecular weights comprising 1 to 10 wt % of high viscosity average molecular weight polyethylene by melting and kneading or multistage polymerization (Japanese Patent Publication (Ibkko-Sho) No. 62-61057), a method for polymerizing polyethylene in which ultra high molecular weight polyethylene having an intrinsic viscosity of 20 dl/g or more is polymerized in an amount of 0.05 or more and less than 1 wt % by multistage polymerization with highly active titanium vanadium solid catalyst component (Japanese Patent Publication (Iokko-Hei) No. 5-79683), and a method for polymerizing polyethylene in which 0.1 to 5 wt % of ultra high molecular weight polyethylene having an intrinsic viscosity of 15 dl/g or more is polymerized by multistage polymerization in a specially arranged polymerization reactor by using a highly active titanium catalyst component preliminarily polymerized with 1-butene or 4-methyl-1-pentene (Japanese Patent Publication (Ibkko-Hei) No. 7-8890).
Furthermore, Japanese Laid-Open Patent Publication (Ibkkai-Hei) No. 5-222122 has disclosed a method for producing polypropylene having a high melt tension by polymerizing propylene by using a preliminarily polymerized catalyst obtained by preliminarily polymerizing ethylene and a polyene compound with a supported titanium-containing solid catalyst component and an organic aluminum compound catalyst component. Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 4-55410 has disclosed a method for producing linear low density polyethylene (LLDPE) having a high melt tension by using a preliminarily polymerized catalyst containing polyethylene having a limiting viscosity of 20 dl/g or more obtained by preliminarily polymerizing ethylene alone with the same catalyst components as above.
Furthermore, the following methods have been proposed in order to enhance a melt tension in the case where a metallocene catalyst type is used: a method of using a catalyst comprising a silica carrier containing at least 1.0 wt % of water, a metallocene, methylaluminoxane and triisobutyl aluminum (Japanese Laid-Open Patent Publication (Tbkkai-Hei) No. 5-140224); a method of using two types of metallocene as catalyst components (Japanese Laid-Open Patent Publication (Tbkkai-Hei) Nos. 5-255436, 5-255437 and 6-206939); and a method of using montmorillonite as a metallocene catalyst type (Japanese Laid-Open Patent Publication (Ibkkai-Hei) Nos. 7-188317 and 7-188336).
In the various proposed compositions and the production methods thereof in connection with the conventional catalyst types, the melt tension of the polyolefin is enhanced to some extent under measurement conditions at 190.degree. C. However, other problems still remain unsolved with respect to the improvement of the melt tension under use conditions at 200.degree. C. or more, a residual odor caused by the crosslinking assistant, the crystallization temperature, the heat stability of properties other than the melt tension, or the like.
Furthermore, although the proposed methods in connection with the metallocene catalyst type provide an improvement of the melt tension of polyolefin under measurement conditions at 190.degree. C., it is still desired to improve the melt tension under use conditions at 200.degree. C. or more.